Brushless motors are widely used in various applications including household electrical appliances and office automation equipment. In a brushless motor, a unit that detects the position of the motor's rotor is provided to acquire timing of when to commutate current applied to each coil. A sensor like, for example, a hall device, is typically used as a unit for this purpose. Using such a sensor will, however, create difficulties in reducing the size and costs of the motor. Thus, an approach is available that makes it possible to detect the position of the rotor, without using any sensor, based on a back electromotive force (back EMF) generated in a coil of a motor.
However, with the approach based on the back EMF, the back EMF cannot be detected while the rotor is in a stationary state, i.e., the motor is stationary, or while the motor is accelerated from the stationary state to a predetermined rotation velocity, and thus it is not possible to acquire timing for commutating the current. To deal with this issue, another approach is available where the rotor is rotated up to a certain velocity at which a back EMF can be detected (see, for example, Japanese Patent No. 3586628 and Japanese Patent Application Laid-open No. H9-233885).
Japanese Patent No. 3586628 discloses a method of starting a motor. More specifically, in the method, a phase of current applied to a coil is commutated regardless of a rotor position, and commutation of phases is repeatedly performed until a back EMF generated is detected.
On the other hand, Japanese Patent Application Laid-open No. H9-233885 discloses another method of starting a motor. More specifically, in the method, current is temporarily applied to a certain coil when the rotor is in a stationary state, whereby the rotor is shifted to a certain position. Then, the current applied is commutated in sequence to accelerate the rotor up to a certain velocity at which a back EMF can be detected.
In the starting method disclosed in Japanese Patent No. 3586628, however, time required for starting a motor cannot be reduced because phases of current applied are repeatedly commutated from the time of a motor start-up until a back EMF is detected.
Also in the starting method disclosed in Japanese Patent Application Laid-open No. H9-233885, time required for starting a motor cannot be reduced because the rotor is shifted to a certain position and then commutation of phases of current applied is performed in sequence until a certain velocity at which a back EMF can be detected.
The present invention is directed to solve such related-art issues. An object of the present invention is to provide a motor starting apparatus and a method of starting a motor to reduce time required for starting the motor.